Most of the plastic ophthalmic lenses sold today by optical dispensers such as retail eyewear outlets are made by machining the desired prescription into the back face of a semi-finished lens blank made from diethylene glycol bis(allyl carbonate) resin, also known as CR-39. These blanks are manufactured off-site by casting the starting monomer for CR-39 between a set of glass molds held together by a flexible gasket and restraints. The mold assembly is initially heated in an oven using a precise cure schedule. During the subsequent polymerization step, the liquid resin is converted into a glassy solid. Shrinkage of up to 16 percent of the material occurs during polymerization and crosslinking. The molds must be designed to account for the shrinkage, so that the lens blank has the desired front curvature. The complexity of design is increased if, instead of a semi-finished lens blank, a finished lens is desired in which both the front and back surfaces have defined curvatures. Another disadvantage in preparing CR-39 lenses is that they require cure schedules of as long as sixteen hours.
Casting lenses from polymerizable compositions on-site would be preferable to a retail eyewear outlet over machining lens blanks if problems associated with shrinkage of the polymerizable material during casting and the long cure time of could be solved. One advantage of casting on-site is that the equipment needed for casting is less expensive than the lens generators and polishing instruments used in lens machining. Second, the casting process is cleaner and generates less waste than the machining process. In addition, the cost of the finished lens to the eyewear outlet using a casting process may be less than that when the lens is prepared by machining a lens blank, particularly for aspheric, multifocal, and progressive lenses.
CR-39 is unsuitable as a material for casting into lenses in one hour processing laboratories because of its slow reaction rate. It would be of great benefit to have a material that maintains most of the desirable properties of CR-39, such as good abrasion resistance, chemical resistance, impact resistance, clarity and generally superior optical properties, yet polymerizes in a short amount of time. It would also be of benefit to have an apparatus that can be used to produce lenses on-site in a short amount of time.
U.S. Pat. No. 4,912,185 to Toh discloses a crosslinkable casting composition for ophthalmic lenses that includes (A) a polyoxyalkylene glycol dimethacrylate or diacrylate, (B) at least one polyfunctional cross-linking agent, and (C) up to 40% by weight of a urethane monomer having from two to six terminal acrylic or methacrylic groups. The polymerizable composition disclosed in the '185 patent was designed to be used in conjunction with traditional methods for the preparation of ophthalmic lenses, wherein the polymerizable solution is poured into numerous molds, cast with blanket radiation, heated and then removed. The method of casting requires the use of a low viscosity polymerizable solution to minimize the problems that result from air entrapment. The '185 patent states that the viscosity of the polymerizable solution should not exceed approximately 200 cps at 25.degree. C. The polyoxyalkylene glycol diacrylate or dimethacrylate, present in the polymerizable composition in a range of 40 to 60% by weight, functions as a viscosity reducing agent for the composition. The polyoxyalkene moieties are based on ethylene oxide or propylene oxide repeating units, with 6 to 11 alkylene oxide repeating units preferred, as shown below. ##STR1##
Methacrylate terminated polyoxyalkylene glycols are preferred over acrylate terminated polyoxyalkylene glycols in the '185 patent because they have lower reactivities than the acrylate counterparts, which, using the traditional casting process, reduces surface aberration and internal stress. The patent indicates that this composition can be fully cured by two to four passes under a UV lamp followed by one hour of heat treatment at 100 degrees C.
An advance in the art of polymerizing shrinkable materials, including polymerizable solutions for ophthalmic lenses, was disclosed in U.S. Ser. No. 07/608,123, entitled "Controlled Casting of a Shrinkable Material," filed on Nov. 1, 1990, that is a continuation-in-part of U.S. Ser. No. 07/345,718, filed on May 1, 1989 by David S. Soane. Briefly, polymerizable material is introduced between two mold halves, one of which is, or both are, constructed of a material that transmits energy, either thermal or UV. Stress related voids in the polymeric material are eliminated by causing the partially polymerized material to polymerize in a differential fashion along a moving front, so that the material ahead of the moving polymer zone remains liquid, and the material that the front has passed is solidified. In a typical method, the moving front is a slit through which UV or thermal energy is transmitted. The still-liquid material ahead of the moving polymer zone can then flow freely, at a rate that equals the rate of shrinkage, and a void-free, reduced stress polymeric network is produced. Using this process, lenses can be cast in a way to prevent cavitation, or voids caused by the shrinkage of material during polymerization. This method is referred to below as "sequential polymerization."
Japanese Patent No. 61064716 (Chem. Abstract 105:192198b) discloses an impact resistant optical resin prepared by polymerizing acrylate or methacrylate, adducts of monoepoxide and brominated bis-phenol, poly-isocyanate and other unsaturated compounds such as styrene or divinylbenzene.
Urethanes have been used in coatings for ophthalmic lenses. U.S. Pat. No. 4,800,123 to Boekeler discloses a scratch resistant coating prepared from a polymerizable composition that includes at least one polyfunctional monomer having three or more acryloloxy groups per molecule, and at least one N-vinyl imido group containing monomer. U.S. Pat. No. 4,435,450 to Coleman discloses a method for applying abrasion resistant thin polyurethane coatings that includes forming a hydroxy-terminated prepolymer which is subsequently crosslinked using a relatively non-volatile triisocyanate, and applying the material by flow coating onto a glass or lens.
Accordingly, it is the object of the present invention to provide a polymerizable composition that can be polymerized into a lens that maintains or exceeds the mechanical and optical properties associated with CR-39, yet has a faster cure rate than CR-39.
It is another object of this invention to provide a polymeric material that can be sequentially polymerized into a finished product in less than one hour, preferably less than thirty minutes.
It is another object of the present invention to provide a polymerizable composition for the production of high quality ophthalmic lenses that is suitable for use in combination with the apparatus and method for sequential polymerization disclosed in U.S. Pat. Nos. 5,110,514 and 5,114,632.
It is still another object of the present invention to provide an apparatus for the production of lenses using the sequential polymerization method.